| GISdevelopment.net ---> Technology ---> Geographic Information System |
Interoperabilty for 3D geodata: Experiences with a deegree-based implementation of CityGML and 3D Web Services
Markus LUPP and Andreas POTH
Abstract
Storage, processing and visualization of 3D geodata became an important subject in the GIS world even before Google introduced its globe viewer. Usage of standards of the Open Geospatial Consortium (OGC) open up new possibilities for combination and usage of 3D geodata. First practical experiences show promising results.
1 Introduction
Processing and visualization of 3D geodata became a common subject during the last years. Some indicators for this are the number of offered software solutions but also the amount of interest for the development of CityGML. CityGML is a GML-based exchange format for three dimensional digital city models, that already is implemented in a number of software products. With the definition of CityGML and application of OGC Web Services (OWS) for access to and visualization of 3D geodata the areas of 3D geodata processing and Spatial Data Infrastructures (SDI) are converging.
This article is discussing solutions that were realized using technology from the deegree Open Source project. The mentioned projects are: “Storage and administration of 3D city models for the cities of Bonn and Berlin”, “Visualization of digital terrain models for the Federal Agency for Cartography and Geodesy of Germany”, “Realization of a transactional CityGML WFS for the Open Geospatial Consortium” and Internet-Visualization of the 3D City model of Bonn”. The experiences from these projects are discussed and chances for the future of 3D SDIs are outlined.
2 OGC-Standards with relevance for 3D
A number of discussion papers and specifications of the OGC are of importance for 3D geodata handling. These are in particular CityGML as a data model and exchange format, KML as a visualization language for earth browsers, Web Feature Service and Web Coverage Service for data access and Web Terrain Service / Web Perspective View Service and Web 3D Service respectively for visualization purposes.
2.1 City Geography Markup Language (CityGML)
CityGML defines a semantic object model for 3D objects in urban areas. It is a GML application schema that is it models objects of an application domain using constructs of the Geography Markup Language. In this aspect CityGML is a semantic model as well as an exchange format.
CityGML was at its beginnings mainly developed by a working group of the SDI initiative of Northrhine-Westfalia, although members from all over Germany were part of this group. In version 0.3 CityGML was introduced into the OGC and published as a discussion paper (Gröger, Kolbe & Czerwinski 2007). The number of contributors to the specification then increased significantly. CityGML 1.0 will soon be an official OGC specification.
2.2 Keyhole Markup Language (KML)
KML is an XML language focused on geographic visualization for earth browsers, including annotation of maps and images. Geographic visualization includes not only the presentation of graphical data on the globe, but also the control of the user's navigation in the sense of where to go and where to look.
Google submitted KML to the OGC to be evolved within the OGC consensus process with KML Version 2.2 being an adopted OGC implementation standard. Future versions may be harmonized with relevant OGC standards that comprise the OGC standards baseline. It is hoped that by bringing KML into the OGC will encourage broader implementation and greater interoperability and sharing of earth browser content and context. Currently, KML 2.2 utilizes a number of geometry elements derived from GML 2.1.2. These elements include point, line string, linear ring, and polygon.
The OGC and Google have agreed that there can be additional harmonization of KML with GML (e.g. to use the same geometry representation) in the future. Consideration of OGC specifications such as Web Map Context, and Symbology Encoding (SE) would make sense in this regard.
There is significant overlap in the functionality described by KML with CityGML and SE, but there also some important differences. The most obvious difference is that KML is a language for visualization purposes for earth browsers, i.e. it consists of both Geometry and styling information and some of this information is specific for the 3D globe use case. Conceptually KML is as a presentation language between GML which is on data model level and a map as e.g. created by a WMS service. It is therefore only logical that the ability to model non-spatial attributes in KML is in comparison poor.
As KML is focussed on 3D visualization it plays a rather important role for display of 3D data, especially because Google Earth reaches a much wider audience than any other geospatial software so far does.
2.3 Web Feature Service
A Web Feature Service (WFS, Vretanos 2004a) allows to query geodata modeled in GML. Filter Encoding (Vretanos 2004b), an SQL-like language encoded in XML is used to create queries to a WFS. A WFS that allows not only to read, but also write access (create, update and delete) is called a transactional WFS (WFS-T).
WFS is an official OGC-standard in the current version 1.1.0. A WFS implementing this 1.1.0 specification has to support GML 3.1.1 – the same version that is the base specification for CityGML. It is therefore possible to use a WFS as a data access layer to CityGML.
2.4 Web Coverage Service
A Web Coverage Service (WCS, Evans 2003) allows to access all kinds of data that is modeled “field-based”, e.g. Raster- or TIN-based. Examples of such data are those created by remote sensing mechnisms or digital terrain models. In the context of 3D SDI a WCS can be used to access terrain models and textures used to be draped on the terrain. WCS is an official OGC standard with the current version 1.1.0.
2.5 Web Terrain Service / Web Perspective View Service
The Web Terrain Service (WTS), still in OGC discussion paper status, generates Views of 3D scenes. In contrast to a WMS that creates 2D visualizations, an image depicting 3D data is generated.
Unfortunately, the development of the WTS specification advanced rather slow during the last years. The current draft version bears the name “Web Perspective View Service” (WPVS) to express that the service is able to depict 3D objects besides “Terrain”. It currently seems that again a critical number of players is interested in developing a standard for a 3D visualization service and it can therefore be hoped that WTS/WPVS as well as W3DS (see 2.6) will become finalized standards in the near future.
Fig. 1: Visualization of terrain and buildings using deegree WTS/WPVS
Fig. 1 shows the result of a WPVS-GetView-request. A digital terrain model is depicted that is textured with aerial photographs. On top of the terrain a number of buildings are displayed (one of them transparently).
WPVS creates presentations of 3D objects. The most important operation of this service is GetView which returns static pictures of 3D landscapes and/or objects. The GetView operation can be seen as an extension to the GetMap operation of WMS. In comparison to GetMap, GetView defines additional parameters allowing to specify a 3D scene. Among these parameters are a rotation angle and the azimuth of the depicted scene. As the result of a GetView operation is a pictures, it is not possible to navigate directly through the scene. A WPVS client is therefore in comparison with real 3D viewers not very interactive, but can be implemented as a web application using (D)HTML without the need for browser plug-ins. Another advantage is that such a simple and web-based 3D client can easily be integrated with other web-client software, like e.g. WMS-based portals.
The challenge when creating a WPVS client is to hide the complexity of a GetView request behind an easy to use graphical user interface, that allows navigation in 3D space.
3 Use cases
The projects mentioned in the introduction need support for the following use cases.
3.1 Storage of digital city models
Digital city models are often created using CAD systems and stored in CAD file formats. This results in a number of disadvantages, it is e.g. not possible to easily select parts of the city model or to organize updates. Because of this reason, organizations who own such city models need homogeneous data that best is stored in a database.
To support this use case it is necessary to store CityGML in a – most likely relational – database. For access to this database a WFS is the obvious choice, CityGML can then directly be inserted and pulled out of the database.
To control the access to the WFS it is necessary to use access control mechanism. In the mentioned projects components of deegree iGeoSecurity are used for this.
3.2 Web Visualization
The advantage of 3D geodata is mostly to be found in its possibilities on visualization. Application areas are support of urban planning processes, town marketing, navigation and others. In the context of planning processes, 3D geo-visualizations allow to display the consequences of planned projects before they are realized.
Tourist information systems can also benefit from 3D visualization. Recognition of landmarks or navigation can be enhanced. Great potential also lies in the coupling of classical 2D maps with 3D scene visualizations.
For marketing purposes of the data itself, terrain and city models are displayed in the Internet. The potential of the data is shown in this way.
3.3 Environmental noise
The EU directive relating to the assessment and management of Environmental noise (2002/49/EG ) obliges the member states in a leveled process to produce strategic noise maps, create action plans as well as to inform the public about noise exposure (Europen Union 2002). For calculation of strategic noise maps (Fitzke 1996) a multitude of geo-information resources are needed in 3D between them:
- Terrain models
- Building models with census data
- Street models including traffic data
- railway models including traffic data
- Noise protection buildings
In Northrine-Westfalia, the processes of data processing and publication are realized using SDI components. (Stöcker-Meier et al. 2007). The digital terrain model is published via deegree Web Coverage Service. So far the experiences with deegree WCS are good without exception.
4 The deegree project
deegree is an Open Source / Free Software project concentrating on components for Spatial Data Infrastructures (SDI). In May 2008 deegree started the OSGeo incubation process and is therefore on its way to become a fully-fledged OSGeo project soon. The Open Source Geospatial Foundatin (OSGeo) is an umbrella organisation for geospatial Open Source projects, its most prominent members probably being UMN Mapserver and GRASS.
In regard to 3D deegree includes all services necessary for a 3D SDI as well as a web-based visualization client and a database model for storing 3D data in relational databases such as Oracle or PostgreSQL/PostGIS.
5 Architecture of a 3D SDI
In the following an architecture is described that was implemented using deegree components and was used as a base for the described exemplar projects.
Fig. 2: Architecture of a 3D SDI
The exemplar architecture displayed in Fig. 2 Uses a geodatabases such as PostGIS or Oracle Spatial for storing building models A transactional WFS (WFS-T) allows read/write access to this data. Access control is implemted using deegree owsProxy so that the data behind the WFS can not be accessed freely. The system used for creation and editing of building models – this usually is a CAD system – accesses the deegree WFS via owsProxy.
Terrain data can also be stored inside the geodatabase, especially if they are not modeled as raster but as TIN or points. Alternatively raster data can be stored in files although the herein described mechanisms for fast access to this data have to be used then. Access to raster data is realized using a WCS. When requesting scenes of the city model it is therefore easily possible to access the corresponding terrain model.
On the right side of Fig. 2 the visualization process is showned. For this deegree-WPVS accesses the terrain data stored in the geodatabase. Alternatively external WFS or WCS service can be used to this end. Beside 3D data textures for the terrain is necessary, e.g. aerial photographs or thematic maps. These can be access using the WMS-interface, in the described projects deegree-WMS is used. A web-based WPVS-client offers a graphical user interface that is usable by common Internet browsers.
6 Conclusion
The development of CityGML defines an important step towards 3D SDIs. The experiences using deegree components for development of such systems that were made in a number of projects are promising. They show that it is already possible to create 3D SDIs using Open Source software.
The mentioned components are available via http://www.deegree.org. At the time of writing the RC3 for WPVS (including a client), WFS and WCS are available as easily installable WAR archives.
7 Bibliography
European Union (2002) Directive 2002/49/EC of the European Parliament and of the Council of 25 June 2002 relating to the assessment and management of environmental noise - Declaration by the Commission in the Conciliation Committee on the Directive relating to the assessment and management of environmental noise . http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32002L0049:EN:NOT Evans, J. (2003) Web Coverage Service (WCS) Version 1.0.0. OpenGIS Project Document 03-065r6 Fitzke, J. (1996) GIS-gestützte Berechnung von Schallimmissionen, in: Proceedings Informatik '96, Klagenfurt, 25.-27. September 1996, http://www.uni-klu.ac.at/groups/geo/gismosim/paper/fitzke/fitzke.htm Gröger, G., T. Kolbe & A. Czerwinski (2007) City Geography Markup Language 0.4.0. OGC project document 07-062.
http://www.opengeospatial.org/standards/gml
Hammes, J. (2001). Modeling of ecosystems as a data source for real-time terrain Rendering, In: Digital Earth Moving : First International Symposium, DEM 2001, Manno, Switzerland, September 5-7, 2001, Proceedings, S. 98ff Müller, M. (2006)(Ed.): Symbology Encoding Implementation Specification. OGC project document 05-077r4.
http://www.opengeospatial.org/standards/symbol Pendergast, M. (2003): Navigate through virtual worlds using Java 3D. JavaWorld.com; 07/04/03; http://www.javaworld.com/javaworld/jw-07-2003/jw-0704-3d_p.html Thiemann, F. (2003): 3D-Gebäude-Generalisierung, in: Kartographische Schriften, Band 7, Visualisierung und Erschließung von Geodaten, Seminar GeoVIS 2003, 27.-28. Februar 2003, Hannover, S. 185 -192.
Vretanos, P.A. (2004a). OpenGIS® Web Feature Service Implementation Specification Version 1.1.0. OpenGIS Project Document 04-094 http://www.opengis.org/specs/?page=specs Vretanos, P.A. (2004b). OpenGIS® Filter Encoding Specification Version 1.1.0. OpenGIS Project Document 04-095.
http://www.opengis.org/specs/?page=specs Stöcker-Meier, E. et al. (2007): Umsetzung der Umgebungslärmrichtlinie aus der Sicht des Landes Nordrhein-Westfalen, In: Lärmbekämpfung - Zeitschrift für Akustik, Schallschutz und Schwingungstechnik, Bd.2. Jg. 2007, Heft Nr.1, S. 7-15.
Quadt, U. & T. Kolbe (2005): Web 3D Service. OGC project document 05-019. https://portal.opengeospatial.org/files/?artifact_id=8869 Watt, A. (2000): 3D computer graphics( third edition).
http://www.citygml.org/
http://www.deegree.org/
http://www.opengeospatial.org
Storage, processing and visualization of 3D geodata became an important subject in the GIS world even before Google introduced its globe viewer. Usage of standards of the Open Geospatial Consortium (OGC) open up new possibilities for combination and usage of 3D geodata. First practical experiences show promising results.
1 Introduction
Processing and visualization of 3D geodata became a common subject during the last years. Some indicators for this are the number of offered software solutions but also the amount of interest for the development of CityGML. CityGML is a GML-based exchange format for three dimensional digital city models, that already is implemented in a number of software products. With the definition of CityGML and application of OGC Web Services (OWS) for access to and visualization of 3D geodata the areas of 3D geodata processing and Spatial Data Infrastructures (SDI) are converging.
This article is discussing solutions that were realized using technology from the deegree Open Source project. The mentioned projects are: “Storage and administration of 3D city models for the cities of Bonn and Berlin”, “Visualization of digital terrain models for the Federal Agency for Cartography and Geodesy of Germany”, “Realization of a transactional CityGML WFS for the Open Geospatial Consortium” and Internet-Visualization of the 3D City model of Bonn”. The experiences from these projects are discussed and chances for the future of 3D SDIs are outlined.
2 OGC-Standards with relevance for 3D
A number of discussion papers and specifications of the OGC are of importance for 3D geodata handling. These are in particular CityGML as a data model and exchange format, KML as a visualization language for earth browsers, Web Feature Service and Web Coverage Service for data access and Web Terrain Service / Web Perspective View Service and Web 3D Service respectively for visualization purposes.
2.1 City Geography Markup Language (CityGML)
CityGML defines a semantic object model for 3D objects in urban areas. It is a GML application schema that is it models objects of an application domain using constructs of the Geography Markup Language. In this aspect CityGML is a semantic model as well as an exchange format.
CityGML was at its beginnings mainly developed by a working group of the SDI initiative of Northrhine-Westfalia, although members from all over Germany were part of this group. In version 0.3 CityGML was introduced into the OGC and published as a discussion paper (Gröger, Kolbe & Czerwinski 2007). The number of contributors to the specification then increased significantly. CityGML 1.0 will soon be an official OGC specification.
2.2 Keyhole Markup Language (KML)
KML is an XML language focused on geographic visualization for earth browsers, including annotation of maps and images. Geographic visualization includes not only the presentation of graphical data on the globe, but also the control of the user's navigation in the sense of where to go and where to look.
Google submitted KML to the OGC to be evolved within the OGC consensus process with KML Version 2.2 being an adopted OGC implementation standard. Future versions may be harmonized with relevant OGC standards that comprise the OGC standards baseline. It is hoped that by bringing KML into the OGC will encourage broader implementation and greater interoperability and sharing of earth browser content and context. Currently, KML 2.2 utilizes a number of geometry elements derived from GML 2.1.2. These elements include point, line string, linear ring, and polygon.
The OGC and Google have agreed that there can be additional harmonization of KML with GML (e.g. to use the same geometry representation) in the future. Consideration of OGC specifications such as Web Map Context, and Symbology Encoding (SE) would make sense in this regard.
There is significant overlap in the functionality described by KML with CityGML and SE, but there also some important differences. The most obvious difference is that KML is a language for visualization purposes for earth browsers, i.e. it consists of both Geometry and styling information and some of this information is specific for the 3D globe use case. Conceptually KML is as a presentation language between GML which is on data model level and a map as e.g. created by a WMS service. It is therefore only logical that the ability to model non-spatial attributes in KML is in comparison poor.
As KML is focussed on 3D visualization it plays a rather important role for display of 3D data, especially because Google Earth reaches a much wider audience than any other geospatial software so far does.
2.3 Web Feature Service
A Web Feature Service (WFS, Vretanos 2004a) allows to query geodata modeled in GML. Filter Encoding (Vretanos 2004b), an SQL-like language encoded in XML is used to create queries to a WFS. A WFS that allows not only to read, but also write access (create, update and delete) is called a transactional WFS (WFS-T).
WFS is an official OGC-standard in the current version 1.1.0. A WFS implementing this 1.1.0 specification has to support GML 3.1.1 – the same version that is the base specification for CityGML. It is therefore possible to use a WFS as a data access layer to CityGML.
2.4 Web Coverage Service
A Web Coverage Service (WCS, Evans 2003) allows to access all kinds of data that is modeled “field-based”, e.g. Raster- or TIN-based. Examples of such data are those created by remote sensing mechnisms or digital terrain models. In the context of 3D SDI a WCS can be used to access terrain models and textures used to be draped on the terrain. WCS is an official OGC standard with the current version 1.1.0.
2.5 Web Terrain Service / Web Perspective View Service
The Web Terrain Service (WTS), still in OGC discussion paper status, generates Views of 3D scenes. In contrast to a WMS that creates 2D visualizations, an image depicting 3D data is generated.
Unfortunately, the development of the WTS specification advanced rather slow during the last years. The current draft version bears the name “Web Perspective View Service” (WPVS) to express that the service is able to depict 3D objects besides “Terrain”. It currently seems that again a critical number of players is interested in developing a standard for a 3D visualization service and it can therefore be hoped that WTS/WPVS as well as W3DS (see 2.6) will become finalized standards in the near future.
Fig. 1: Visualization of terrain and buildings using deegree WTS/WPVS
Fig. 1 shows the result of a WPVS-GetView-request. A digital terrain model is depicted that is textured with aerial photographs. On top of the terrain a number of buildings are displayed (one of them transparently).
WPVS creates presentations of 3D objects. The most important operation of this service is GetView which returns static pictures of 3D landscapes and/or objects. The GetView operation can be seen as an extension to the GetMap operation of WMS. In comparison to GetMap, GetView defines additional parameters allowing to specify a 3D scene. Among these parameters are a rotation angle and the azimuth of the depicted scene. As the result of a GetView operation is a pictures, it is not possible to navigate directly through the scene. A WPVS client is therefore in comparison with real 3D viewers not very interactive, but can be implemented as a web application using (D)HTML without the need for browser plug-ins. Another advantage is that such a simple and web-based 3D client can easily be integrated with other web-client software, like e.g. WMS-based portals.
The challenge when creating a WPVS client is to hide the complexity of a GetView request behind an easy to use graphical user interface, that allows navigation in 3D space.
3 Use cases
The projects mentioned in the introduction need support for the following use cases.
3.1 Storage of digital city models
Digital city models are often created using CAD systems and stored in CAD file formats. This results in a number of disadvantages, it is e.g. not possible to easily select parts of the city model or to organize updates. Because of this reason, organizations who own such city models need homogeneous data that best is stored in a database.
To support this use case it is necessary to store CityGML in a – most likely relational – database. For access to this database a WFS is the obvious choice, CityGML can then directly be inserted and pulled out of the database.
To control the access to the WFS it is necessary to use access control mechanism. In the mentioned projects components of deegree iGeoSecurity are used for this.
3.2 Web Visualization
The advantage of 3D geodata is mostly to be found in its possibilities on visualization. Application areas are support of urban planning processes, town marketing, navigation and others. In the context of planning processes, 3D geo-visualizations allow to display the consequences of planned projects before they are realized.
Tourist information systems can also benefit from 3D visualization. Recognition of landmarks or navigation can be enhanced. Great potential also lies in the coupling of classical 2D maps with 3D scene visualizations.
For marketing purposes of the data itself, terrain and city models are displayed in the Internet. The potential of the data is shown in this way.
3.3 Environmental noise
The EU directive relating to the assessment and management of Environmental noise (2002/49/EG ) obliges the member states in a leveled process to produce strategic noise maps, create action plans as well as to inform the public about noise exposure (Europen Union 2002). For calculation of strategic noise maps (Fitzke 1996) a multitude of geo-information resources are needed in 3D between them:
- Terrain models
- Building models with census data
- Street models including traffic data
- railway models including traffic data
- Noise protection buildings
In Northrine-Westfalia, the processes of data processing and publication are realized using SDI components. (Stöcker-Meier et al. 2007). The digital terrain model is published via deegree Web Coverage Service. So far the experiences with deegree WCS are good without exception.
4 The deegree project
deegree is an Open Source / Free Software project concentrating on components for Spatial Data Infrastructures (SDI). In May 2008 deegree started the OSGeo incubation process and is therefore on its way to become a fully-fledged OSGeo project soon. The Open Source Geospatial Foundatin (OSGeo) is an umbrella organisation for geospatial Open Source projects, its most prominent members probably being UMN Mapserver and GRASS.
In regard to 3D deegree includes all services necessary for a 3D SDI as well as a web-based visualization client and a database model for storing 3D data in relational databases such as Oracle or PostgreSQL/PostGIS.
5 Architecture of a 3D SDI
In the following an architecture is described that was implemented using deegree components and was used as a base for the described exemplar projects.
Fig. 2: Architecture of a 3D SDI
The exemplar architecture displayed in Fig. 2 Uses a geodatabases such as PostGIS or Oracle Spatial for storing building models A transactional WFS (WFS-T) allows read/write access to this data. Access control is implemted using deegree owsProxy so that the data behind the WFS can not be accessed freely. The system used for creation and editing of building models – this usually is a CAD system – accesses the deegree WFS via owsProxy.
Terrain data can also be stored inside the geodatabase, especially if they are not modeled as raster but as TIN or points. Alternatively raster data can be stored in files although the herein described mechanisms for fast access to this data have to be used then. Access to raster data is realized using a WCS. When requesting scenes of the city model it is therefore easily possible to access the corresponding terrain model.
On the right side of Fig. 2 the visualization process is showned. For this deegree-WPVS accesses the terrain data stored in the geodatabase. Alternatively external WFS or WCS service can be used to this end. Beside 3D data textures for the terrain is necessary, e.g. aerial photographs or thematic maps. These can be access using the WMS-interface, in the described projects deegree-WMS is used. A web-based WPVS-client offers a graphical user interface that is usable by common Internet browsers.
6 Conclusion
The development of CityGML defines an important step towards 3D SDIs. The experiences using deegree components for development of such systems that were made in a number of projects are promising. They show that it is already possible to create 3D SDIs using Open Source software.
The mentioned components are available via http://www.deegree.org. At the time of writing the RC3 for WPVS (including a client), WFS and WCS are available as easily installable WAR archives.
7 Bibliography
European Union (2002) Directive 2002/49/EC of the European Parliament and of the Council of 25 June 2002 relating to the assessment and management of environmental noise - Declaration by the Commission in the Conciliation Committee on the Directive relating to the assessment and management of environmental noise . http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32002L0049:EN:NOT Evans, J. (2003) Web Coverage Service (WCS) Version 1.0.0. OpenGIS Project Document 03-065r6 Fitzke, J. (1996) GIS-gestützte Berechnung von Schallimmissionen, in: Proceedings Informatik '96, Klagenfurt, 25.-27. September 1996, http://www.uni-klu.ac.at/groups/geo/gismosim/paper/fitzke/fitzke.htm Gröger, G., T. Kolbe & A. Czerwinski (2007) City Geography Markup Language 0.4.0. OGC project document 07-062.
http://www.opengeospatial.org/standards/gml
Hammes, J. (2001). Modeling of ecosystems as a data source for real-time terrain Rendering, In: Digital Earth Moving : First International Symposium, DEM 2001, Manno, Switzerland, September 5-7, 2001, Proceedings, S. 98ff Müller, M. (2006)(Ed.): Symbology Encoding Implementation Specification. OGC project document 05-077r4.
http://www.opengeospatial.org/standards/symbol Pendergast, M. (2003): Navigate through virtual worlds using Java 3D. JavaWorld.com; 07/04/03; http://www.javaworld.com/javaworld/jw-07-2003/jw-0704-3d_p.html Thiemann, F. (2003): 3D-Gebäude-Generalisierung, in: Kartographische Schriften, Band 7, Visualisierung und Erschließung von Geodaten, Seminar GeoVIS 2003, 27.-28. Februar 2003, Hannover, S. 185 -192.
Vretanos, P.A. (2004a). OpenGIS® Web Feature Service Implementation Specification Version 1.1.0. OpenGIS Project Document 04-094 http://www.opengis.org/specs/?page=specs Vretanos, P.A. (2004b). OpenGIS® Filter Encoding Specification Version 1.1.0. OpenGIS Project Document 04-095.
http://www.opengis.org/specs/?page=specs Stöcker-Meier, E. et al. (2007): Umsetzung der Umgebungslärmrichtlinie aus der Sicht des Landes Nordrhein-Westfalen, In: Lärmbekämpfung - Zeitschrift für Akustik, Schallschutz und Schwingungstechnik, Bd.2. Jg. 2007, Heft Nr.1, S. 7-15.
Quadt, U. & T. Kolbe (2005): Web 3D Service. OGC project document 05-019. https://portal.opengeospatial.org/files/?artifact_id=8869 Watt, A. (2000): 3D computer graphics( third edition).
http://www.citygml.org/
http://www.deegree.org/
http://www.opengeospatial.org